Automatic apparatus for performing ELISA tests are commercially available. Such apparatus perform the tests on supports having a plurality of cells or wells for facilitating parallel and orderly processing of the samples. These supports, which are known as micro-titer plates, usually have a format of 8.0 times. 12 wells, for a total of 96 wells, and includes a robot capable of moving in X, Y and Z coordinates for picking and placing a sample or reagent tip, e.g., a pipette tip, in a well. However, occasionally, the apparatus may miss picking a sample or reagent tip. Since sample or reagent tips are essentially transparent to visible light, it is difficult to determine when a sample or reagent tip is missing.